1. Field of the Invention
The present invention relates to an apparatus for preventing corrosion of a contact, which flows a large current to thereby destroy an oxide layer developed on the contact such as a switch or a connector and preventing the corrosion.
2. Description of the Related Art
Inputs for various electronic controls have often been connected to a contact such as a switch or a connector. For example, in order to perform various types of control for an automobile, it is necessary to give input signals for control to many electronic control units (ECU). Input signals are given to an input terminal of an electronic control unit from a contact of a switch, which mechanically opens and closes, via a contact of a connector.
Contacts such as a switch and a connector have been made with metal materials excellent in electric conduction so as to reduce contact resistance in electric connection. These contacts may increase in contact resistance because a surface of a contact part is oxidized during electric disconnection. Further, a surface of a part exposed around the contact part may be oxidized to produce an oxide and then, the oxide may be caught in the contact part, resulting in an increased contact resistance. Even if the contact is oxidized to increase the contact resistance, when a contact state and a non-contact state are appropriately repeated and a relatively large current flows in the contact state, heat generated by the current removes the oxide, so that the increase of the contact resistance can be prevented.
However, with regard to input into an electronic appliance, it is in general not necessary to flow a large current capable of preventing corrosion constantly into contacts. An intermittent flow of such a large current may contribute to malfunctions due to noise. In addition, flowing a large current into a contact may deteriorate electric life of the contact largely or may cause adhesion of the contact. In order to solve these problems, JP-A-Hei. 2-297818 discloses an apparatus for controlling a current flowing into contacts. The apparatus detects a contact resistance of the contact, and flows a large current into between the contacts when the detected contact resistance is equal to or larger than a predetermined reference value.
Also, U.S. Pat. No. 5,523,633 discloses a circuit for preventing corrosion of a switch for large current. The switch allows a large current in a pulse shape during a period in which a contact of the switch is turned on, when the switch for large current is employed in a low-current system such as electronic control units. In addition, JP-A-Hei. 7-14463 discloses a device for discriminating contact signals. The device allows a corrosion-prevention current in a pulse shape to flow periodically by means of charge and discharge into a condenser. JP-A-2002-343171 also discloses a device for preventing corrosion of a contact of a switch. The device flows large current for preventing corrosion for at least a predetermined holding time from a time point where the contact of the switch is changed from an opened state to a closed state. When the contact of the switch is in the opened state, the device decreases an impedance of an input signal line connected to the contact.